In December 2017 the ICAS the professional body of Chartered Accountants listed the top 30 accountancy firms:
Deloitte
PwC
EY
KPMG
BDO
Grant Thornton etc etc
In June 2017 The Financial Times reported:
"FRC finds audit shortcoming at three big accountancy firms
"KPMG, Deloitte and Grant Thornton are lagging behind their professional services peers in terms of the quality of their audits, according to an annual review by the UK’s accounting watchdog.
"The Financial Reporting Council said on Thursday that of the 23 individual KPMG audits it reviewed between February 2016 and January 2017, two required “significant improvements”, while for Deloitte, another of the Big Four accountancy firms, that figure was also two out of 23. Deloitte had a higher proportion of audits that the FRC deemed “good”, however.
Smaller rival Grant Thornton received the worst results; out of only eight audits reviewed, three were said to need significant improvements. For BDO, this was one out of eight.
By contrast, none of the EY or PwC audits reviewed by the watchdog fell into that category.
"Recent high-profile accounting scandals at UK companies such as Rolls-Royce and BT — which have involved KPMG and PwC, respectively — have raised questions over whether auditors inspect company accounts with appropriate scepticism. Toughening its approach to accounting misconduct, the FRC recently fined PwC a record £5m for “misconduct” relating to its audit of social housing group Connaught, and in November imposed a £4m penalty on Deloitte for its audits of Aim-listed Aero."
More recently, in January this year the Guardian reported:
"Calls for more scrutiny of top accounting firms after Carillion collapse
"Head of industry’s watchdog says competition authorities should examine the sector
"MPs on the work and pensions select committee were told auditors and Carillion were ‘all mates’.
"Britain’s four biggest accountancy companies are facing fresh scrutiny, with the head of the industry watchdog calling for the competition regulator to investigate their auditing activities following the collapse of Carillion.
"Stephen Haddrill, the chief executive of the Financial Reporting Council (FRC), told MPs at a joint select committee hearing on Tuesday that “there should be more competition in the major accounting and audit area”.
This suggests to me that not everything is quite as cut and dried and above board as you say primrose.